First of all, let me say right off the top, I'm not going to tell you what I think you should do. As always, it's your choice. As for me, I generally wear one. I usually skip it on short local trips and on the bike path, but always wear one riding in traffic.
A little back story: I've had two accidents where wearing a helmet has saved me from...well I don't know from what, obviously...but certainly things would have been worse. Both involved a "right cross" scenario. Once I went over (wasn't clipped in) and landed on my head on the other side of the car. Once I was clipped in, and my head was slammed down on top of the hood.
Yes, I know the Euro-types almost never do, but keep in mind, Europe is not America. In Amsterdam, for example, you have bike paths that will take you anywhere -- all separated from vehicular traffic. I likely wouldn't wear a helmet there either (and not because of peer pressure).
In America you inevitably are out there jockeying for position with 2-Ton SUV's. Vulnerable is an understatement. Additionally I would say drivers state-side are in general much less tolerant of cyclists, and much less prepared for sharing the road with them.
Thankfully there are helmets from companies like Nutcase (that's my helmet in the thumbnail) that don't look quite so alien. The Nutcase motto is "I love my brain," which is something to think about. But as always (and despite what the lawyers will tell you), the responsibility and the choice is yours alone.
I would argue that experience and awareness play much more of a role than any sort of protective equipment. But me personally, when I'm out in traffic with the assholes, I want every advantage I can get -- however slight.
And on a lighter note, here's a cartoon that elucidates a somewhat different point of view.
[image via Nutcase]